Harmonic sophistication is a...fraught term. There were jazz musicians who developed a style based on 1 chord, but it needn't be harmonically simple. Ever tried riffing off a humming appliance? It's really fun. I used to do it while waiting for my laundry, since the washers all hummed at the same pitch. When it's you and a fixed tone, your harmonic palette starts opening up in wild ways. Microtones creep in, and new decoration schemes.
You also have to be careful because Bach and Purcell didn't think in terms of our modern harmonic structure. They worked in voicing and figured base, which produces similar, but not identical, results. Go back a little farther, and it's all voicing. Palestrina in in some ways closer to Schoenberg than Haydn.
As for the breakdown of harmony in art music, not so much. Serialism got all the academic and theoretical attention, and many mediocre composers with a theoretical bent worked in it, and a few great composers like Stravinsky and Shostakovitch, but it's hardly all the art music that was being composed. Many composers got through their whole careers without any particular use of it, such as Walter Piston. What that has to do with rock and roll is a mystery to me, since it's an offshoot of jazz, which is alive and well and still serving as a wellspring for new music.
You also have to be careful because Bach and Purcell didn't think in terms of our modern harmonic structure. They worked in voicing and figured base, which produces similar, but not identical, results. Go back a little farther, and it's all voicing. Palestrina in in some ways closer to Schoenberg than Haydn.
As for the breakdown of harmony in art music, not so much. Serialism got all the academic and theoretical attention, and many mediocre composers with a theoretical bent worked in it, and a few great composers like Stravinsky and Shostakovitch, but it's hardly all the art music that was being composed. Many composers got through their whole careers without any particular use of it, such as Walter Piston. What that has to do with rock and roll is a mystery to me, since it's an offshoot of jazz, which is alive and well and still serving as a wellspring for new music.